Causes

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The cause of psoriasis isn't fully known, but it's thought to be related to an immune system problem with cells in your body. More specifically, one key cell is a type of white blood cell called a T lymphocyte or T cell. Normally, T cells travel throughout the body to detect and fight off foreign substances, such as viruses or bacteria. If you have psoriasis, however, the T cells attack healthy skin cells by mistake, as if to heal a wound or to fight an infection.

Overactive T cells trigger other immune responses. The effects include dilation of blood vessels in the skin around the plaques and an increase in other white blood cells that can enter the outer layer of skin. These changes result in an increased production of both healthy skin cells and more T cells and other white blood cells. This causes an ongoing cycle in which new skin cells move to the outermost layer of skin too quickly — in days rather than weeks. Dead skin and white blood cells can't slough off quickly enough and build up in thick, scaly patches on the skin's surface. This usually doesn't stop unless treatment interrupts the cycle.

Just what causes T cells to malfunction in people with psoriasis isn't entirely clear. Researchers have found genes that are linked to the development of psoriasis, but environmental factors also play a role.

Psoriasis triggers

Psoriasis typically starts or worsens because of a trigger that you may be able to identify and avoid. Factors that may trigger psoriasis include:

Infections, such as strep throat or skin infections

Injury to the skin, such as a cut or scrape, bug bite, or a severe sunburn

Stress

Cold weather

Smoking

Heavy alcohol consumption

Certain medications — including lithium, which is prescribed for bipolar disorder; high blood pressure medications such as beta blockers; antimalarial drugs; and iodides.

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